The supposed amount of time (5 seconds)you have to pick up food after it's been dropped on the ground or floor. Allegedly, the bacteria doesn't get on it before 5 seconds is up so you can still eat it if you pick it up really quickly. Sadly, it's all bullshit and not true but it makes good for justification - mainly so people don't call you gross.

*drops an ice lolly*
"FIVE SECOND RULE!"
*picks it up, brushes it off, sticks in mouth*

You know when you have some fried chicken on your plate and it's the last piece at the cookout and of COURSE you drop it on the floor. Then what do you do? After all, it's disgusting to eat food that fell on the floor. Well, the 5 SECOND RULE says that after 5 seconds, any food that fell on the floor immediately becomes un-germified. So after 5 seconds, eat whatever you dropped on the floor, cuz the rule is what really matters.